This invention relates generally to switching power transformer circuits. More particularly, it relates to such circuits that employ pulse width modulation of a signal that approximates a square wave for regulation of output voltage.
The invention may advantageously be used with power supplies, particularly D.C.-to-D.C. convertors of the half-bridge, full-bridge, or push-pull type, although it is also suitable for numerous other transformer circuit applications.
Conventional half-bridge, full-bridge and push-pull convertors, although widely used, suffer from a tendency toward power transformer core saturation. This saturation results from unswitched D.C. currents flowing in the primary or secondary windings. This core saturation may lead to catastrophic failure or erratic circuit operation. The undesirable saturation may result from power transistor mismatch, rectifier forward voltage mismatch, winding resistance mismatch, or, in bridge-type circuits, from D.C. input voltage differences caused by input filter capacitor mismatch or other anomalies in the input circuit.
In certain prior art circuits a D.C. blocking capacitor is inserted in series with the power transformer primary winding. Other prior art circuits rely upon matching of power transistors or upon air gaps in the power transformer core, or upon both of these techniques. Some prior art circuits require the use of stepped gaps in the power transformer core along with an additional winding on the transformer to sense the effects of D.C. current. Still other circuits alter the voltage regulation feedback circuit in such a way to change the convertor operation to simulate a current-fed type.
The solutions attempted by the prior art noted above all suffer from various disadvantages. D.C. blocking capacitors generally are applicable only to the bridge type of convertors, and their use may cause unequal voltage stress on output rectifiers. Such a capacitor must conduct the full power transformer primary current and may thus be unreliable. Also, more than one capacitor may be required in high current applications. The matching of power transistors, while addressing the problem, is both costly and unreliable. The use of transformer air gaps decreases the power transformer primary inductance, thus increasing maximum current that must be conducted by the power transistors. Also, air gaps increase transformer leakage flux that may interfere with other portions of the circuitry. Complex transformer designs, such as the use of stepped gaps in the power transformers are expensive and difficult to design and tie the transformer design very closely to that of the control circuit, requiring custom designing for each different circuit. Alteration or manipulation of the voltage regulation feedback control circuits may cause difficulties when the circuit is operating under light loads or in the presence of line and load transient conditions, and they usually require additional circuitry to correct for these weaknesses. Such control circuit manipulation also must be designed in close relationship to the voltage control loop and may thus make the control loop difficult to optimize for operation and difficult to analyze.